docs: def on_raise(self, node):    # ('type', 'inst', 'tback')
          """Raise statement: note difference for python 2 and 3."""
          if version_info[0] == 3:
              excnode = node.exc
              msgnode = node.cause
          else:
              excnode = node.type
              msgnode = node.inst
          out = self.run(excnode)
          msg = ' '.join(out.args)
          msg2 = self.run(msgnode)
          if msg2 not in (None, 'None'):
              msg = "%s: %s" % (msg, msg2)
          self.raise_exception(None, exc=out.__class__, msg=msg, expr='')